A membrane solvent extraction process is disclosed in Lee et al U.S. Pat. No. 3,956,112 as an improvement over conventional solvent extraction. Lee et al describes the membrane solvent extraction system as a process comprising the steps of contacting one side of a polymeric substantially non-porous membrane with a feed solvent liquid B containing a solute material A and contacting the other side of the membrane with an extracting solvent liquid C which is substantially immiscible with liquid B, the membrane being swollen by the solvents thereby forming an intermediary zone and allowing diffusion through the swollen membrane of the solute A while preventing direct phase to phase contact between solvents B and C. Diffusivity of A in the membrane between the feed solvent and the extracting solvent is specified in terms of Fick's equation as being in the range of about 1.times.10.sup.-9 to about 1.times.10.sup.-4 cm.sup.2 /sec. In this system the removal of solute A from the feed solvent liquid into the extracting solvent liquid C depends not only on diffusivity but on the relative solubilities of the solute A in the solvents B and C, i.e., to obtain substantial removal of solute A from feed solvent B the solubility of solute A in the extracting solvent C must be substantially greater than in the feed solvent B or large volumes of extracting solvent C must be used; i.e., the distribution of solute A between the feed solvent and the extracting solvent will not go beyond the partition coefficient of solute A.
As background with respect to a preferred embodiment of this invention, the conversion of cyclohexane into adipic acid by processes comprising liquid phase air oxidation of cyclohexane to a mixture comprising cyclohexanol, cyclohexanone and unreacted cyclohexane, followed by separation of the unreacted cyclohexane, and nitric acid oxidation of the cyclohexanol and cyclohexanone mixture to give adipic acid is disclosed in U.S. Pat. Nos. 2,439,513 and 2,557,282. Such processes have provided a commercially valuable route to adipic acid, which is an ingredient for the production of nylon.
Polymerization of adipic acid with hexamethylenediamine yields polyhexamethyleneadipamide, i.e. nylon, which is useful in various textile and plastic applications, and is manufactured on a scale of many millions of pounds per year. Therefore, processes for the production of adipic acid which are more economical are commercially valuable. The processes taught in the above patents provide very low yields of adipic acid from cyclohexane, thus resulting in excessive capital investment, excessive usage of energy during processing and excessive consumption of raw materials, particularly cyclohexane. In particular, excessive energy is required for separating the cyclohexane from the cyclohexanol and cyclohexanone by the separation processes, e.g., steam distillation, taught therein. It is clear from a careful reading of the above patents that membrane separation processes were not contemplated by the patentees.
In the processes disclosed in the above patents, the cyclohexane oxidation step is normally carried out to low levels of cyclohexane conversion, e.g., about 6% to 8%, to minimize degradation of cyclohexanol and cyclohexanone to products unsuitable for the following nitric acid oxidation step. One particular embodiment of the instant invention avoids this problem by continuously removing the cyclohexane oxidation products, e.g., cyclohexanol and cyclohexanone, as they are formed.
Processes for continuous separation of reaction products of organic chemical reactions are known in the art. See, for example, U.S. Pat. No. 2,956,071, which teaches a process for continuously removing water, which is formed as a reaction product from organic chemical reactants, e.g., as in esterification reactions. This process utilizes hydrophilic membranes to continuously remove water and drive the reaction to completion. This process differs from the process of the instant invention in substantial ways. For example, the patented process contemplates the use of cation exchange materials as catalysts for esterification rather than the conventional strong acid esterification catalysts, e.g. sulfuric acid, which would attack the membrane. In the process of the instant invention, this problem may be avoided by careful selection of the membrane material. Thus in one preferred embodiment, a fluorosulfonic acid polymer membrane is utilized in a strong acid environment, thus avoiding the problems relating to use of a cation exchange material as a catalyst.
The instant process also differs in other substantial ways. For example, unlike the prior art process, the instant process is driven by continuous reaction as well as diffusion, i.e. the process proceeds by reacting a compound which has crossed the membrane and thus maintaining a high concentration gradient.